1. Field of the Invention
The present invention relates to a method of determining a threshold array for generating a gradation image, suitable for use in apparatus in printing applications, such as a color scanner, an image setter, a CTP (Computer To Plate) apparatus, a CTC (Computer To Cylinder) apparatus, a DDCP (Direct Digital Color Proof) apparatus, or the like.
2. Description of the Related Art
Halftone image output apparatus such as an image setter or the like produce a halftone image (gradation image) of binary pixels (e.g., black and white pixels generated by turning on and off an applied laser beam) on a printing sheet or a film. It has been pointed out that such a halftone image output apparatus suffers moiré patterns generated on outputted images due to the conflict or interference between its output resolution and screen ruling (see Japanese Laid-Open Patent Publication No. 8-317212).
The output resolution refers to the resolution of the image output apparatus, and is defined by dpi (dots per inch), pixels/inch (same as dpi), or pixels/mm (represented by dpmm or lpmm). The screen ruling refers to lines/inch (may be converted into lines/mm) which represents the number of columns of dots (also referred to as dot cells) contained in a unit length (one inch), and is defined by lpi (lines per inch) and also called screen frequency or dot frequency.
A moiré pattern generated due to the interference between an output resolution and a screen ruling is a periodic pattern of dots, i.e., a periodic interference fringe pattern produced between the dot pitch and the scanning line pitch. The moiré pattern serves as a low-frequency noise component and lowers the quality of the produced image.
The inventor of the present application has proposed techniques for reducing such a low-frequency noise component in Japanese Laid-Open Patent Publication No. 11-112814 (hereinafter referred to as “first technique”) and Japanese Patent Application No. 2001-28838 (hereinafter referred to as “second technique”).
According to the first technique, among existing thresholds to be corrected in a threshold array (also referred to as “threshold template”), a threshold to be corrected is compared with a central value within a given threshold correcting range and converted into dot image data, which is then converted into data in a frequency space. From the data in the frequency space, there is extracted data containing a low-frequency noise component whose frequency is lower than the basic frequency of dots, and the extracted data is converted into image data in an actual space. The converted image data in the actual space and the threshold to be corrected are observed within the given threshold correcting range, and a pair of thresholds to be replaced (basically, a pair of thresholds located in positions for generating pixels having maximum and minimum values of the image data in the actual space) is selected and replaced, thus producing a corrected threshold array.
The corrected threshold array produced by the first technique is resistant to the generation of a low-frequency noise component.
According to the second technique, an existing threshold array is not corrected, but a threshold array is newly generated which makes an outputted gradation image more resistant to moiré. The second technique is high in freedom and has an increased ability to reduce moiré.
The first and second moiré reduction techniques are effective when applied to halftone images having a relatively high resolution of 2400 dpi and a relatively high screen ruling of 175 lpi, for example.
Specifically, the first and second moiré reduction techniques are suitable for being applied to threshold arrays for generating halftone images whose number of pixels per dot (also referred to as dot number which is calculated by (2400/175)2 in the above example, about 188) is relatively large.
Under the conditions of 2400 dpi and 175 lpi, however, apparatus in printing applications, such as a color scanner, an image setter, a CTP apparatus, a CTC apparatus, a DDCP apparatus, etc. can output images of desired quality, but have to process an increased amount of image data and need a long period of time required to process and output image data.
The inventor of the present application has found that under the conditions in which the output resolution and the screen ruling are more liable to interfere with each other, tending to produce moiré (single-plate moiré), e.g., under the output conditions of 1200 dpi and 175 lpi, or generally under the output conditions in which the ratio of the output resolution (dpi)/the screen ruling (lpi) is equal to or smaller than 10, the proportion of one pixel in a dot increases, resulting in a large quantizing error, and moiré tends to remain unremoved even according to the first and second techniques.
Actually, an image outputted under the conditions of 1200 dpi and 175 lpi and an image outputted under the conditions of 2000 dpi and 175 lpi are made up of pixels having respective sizes of about 21 μm and 13 μm which are too small for the human eye to distinguish between their resolutions.
Therefore, if an image outputted under the conditions of 1200 dpi and 175 lpi, which are subject to a greater quantizing error than the conditions of 2000 dpi and 175 lpi, is free of a moiré pattern, then apparatus in printing applications, such as a color scanner, an image setter, a CTP apparatus, a CTC apparatus, a DDCP apparatus, etc. can be made simpler in arrangement and can be operated at higher processing speeds.